1. Field of the Invention
The present invention generally relates to methods of forming high dielectric constant thin films, and more specifically to a method of forming a high-dielectric constant thin film used for a semiconductor memory or the like. The present invention also relates to a method of manufacturing a semiconductor device provided with such high dielectric constant thin film.
2. Description of the Background Art
Recently, semiconductor memories or devices have been increasingly reduced in size. In a dynamic random access memory (a DRAM), for example, the number of bits has quadrupled in three years. Such rapid reduction in size is intended for high integration degree of the device, reduction in power consumption and cost, or the like. However, even if the integration degree increases, a capacitor of the DRAM must be provided with a prescribed capacitance. Therefore, a thickness of a film of a capacitor material must be reduced. With SiO.sub.2 which has conventionally been used, it is difficult to reduce the thickness of the film. If the material provided with higher dielectric constant is employed, a sufficient capacitance would be ensured as in the case of the reduction in the thickness of the film. Thus, many studies have been done for utilizing a high dielectric constant material for the memory device.
The most important requirement for the capacitor material is that it is a thin film having the above mentioned high dielectric constant and small leakage current. Namely, as long as the high dielectric constant material is used, a thickness of the film must be as small as possible and the leakage current must be a minimum value. Generally, it is preferable that the thickness of the film is at most 0.5 nm of an equivalent SiO.sub.2 thickness and a leakage current density when 1 V is applied is at most 2.times.10-.sup.7 A/cm.sup.2. In the case of the thin film formed on an electrode for a capacitor of the DRAM having a step portion, the film must be formed by CVD (Chemical Vapor Deposition) providing high conformity even in the case of a complicated shape. However, there is no such material for CVD that has stable and sufficient vaporization property. This is because a .beta.-diketone type dipivaloylmethanate (hereinafter abbreviated as DPM) compound, which is often used as the material for CVD, is not provided with sufficient vaporization property by heating.
In this context, the present inventor has found that the effect of the vaporization property dramatically increases when a conventional solid material is dissolved in an organic solvent of tetrahydrofuran (hereinafter abbreviated as THF), and proposed that the material should be used for CVD (Japanese Patent Laying-Open No.5-299365). However, it has been found that a good result cannot necessarily be obtained even when the material is used for formation of a dielectric film using a CVD apparatus for a liquid material which has conventionally been used for forming an SiO.sub.2 film. Then, the present inventor has developed a CVD apparatus for a liquid material capable of sufficiently vaporizing the liquid material and stably supplying it for a reaction chamber (Japanese Patent Laying-Open No. 6-310444 and No. 7-94426). However, even when the dielectric film is formed by the CVD apparatus for vaporizing the solution, stable and sufficient film property (including electrical property) is not necessarily obtained.
FIG. 5 is a schematic illustration showing a CVD apparatus for forming a high dielectric constant thin film which has been disclosed in Japanese Patent Laying-Open No. 9-219497.
Referring to FIG. 5, a solution 14 in which a solid of Ba (DPM).sub.2 is dissolved in a solvent of THF (the solution is hereinafter abbreviated as Ba (DPM).sub.2 /THF), Sr (DPM).sub.2 /THF 15 and Ti (t-BuO).sub.2 (DPM).sub.2 /THF 16 are fed to a vaporizer 23 through liquid mass flow controllers 19 and air valves 22. THF 17 is fed to vaporizer 23 through liquid mass flow controllers 19 and air valves 21. N.sub.2 carrier gas 18 is fed to vaporizer 23 through gas flow controller 20 and air valve 21. Vaporizer 23 is provided above a reaction chamber 32. A susceptor 30 with a heater 31 is provided in reaction chamber 32. A substrate 29 is placed on susceptor 30. A gas nozzle 28 is provided on a ceiling of reaction chamber 32. The material gas which has been fed to vaporizer 23 is mixed with O.sub.2 gas 33 in a mixer 27, and supplied for reaction chamber 32 through gas nozzle 28. A vent is denoted by a reference numeral 26, and air valves on the sides of vent and reaction chamber are respectively denoted by 24 and 25. A pressure controller 34 is provided below reaction chamber 32.
Now, a method of manufacturing a high dielectric constant thin film using a conventional CVD apparatus for forming the high dielectric constant thin film will be described.
Referring to FIG. 5, a mixture of Ba (DPM).sub.2 /THF 14, Sr (DPM)2THF 15, Ti (t-BuO).sub.2 (DPM).sub.2 /THF 16, THF 17 and N.sub.2 carrier gas 18 are fed to vent 26 from vaporizer 23, so that a flow of the mixture of the gas and liquid is obtained. O.sub.2 gas 33 is supplied for reaction chamber 32 and pressure controller 34 is adjusted, so that a pressure in reaction chamber 32 is set at a desired level (for example at 5 Torr). Air valve for vent 24 is closed and, at the same time, air valve for reaction chamber 25 is opened. Thus, the mixture of gas and liquid is supplied for reaction chamber 32, and a film of (Ba, Sr) TiO.sub.3 (hereinafter abbreviated as a BST thin film) of Stoichiometric is formed on substrate 29.
FIG. 6 is a cross sectional view showing a semiconductor device thus formed. Referring to FIG. 6, a poly-Si plug 4 is formed in a silicon substrate 2. A barrier metal 3 is formed on poly-Si plug 4. An Ru storage node 1 is formed on barrier metal 3. A BST thin film 35 including TiO (DPM).sub.2 is formed to cover Ru storage node 1. When thicknesses 6 and 7 of BST thin film 35 formed on Ru storage node 1 are respectively d.sub.max and d.sub.min, a distance 8 between adjacent Ru storage nodes 1, 1 is W (.about.0.13 .mu.m) and a height 9 of Ru storage node 1 is D (.about.0.36 .mu.m), a coverage d.sub.min /d.sub.max is at most 50% for an aspect ratio D/W (at least 3).
The conventional method of manufacturing the high dielectric constant thin film has been performed as described above.
However, in the case of the DRAM of 4-Gbit class or the like, aspect ratio D/W is at least 3 for the semiconductor device having a step portion as shown in FIG. 6. Even on the step portion, a conformal coverage (at least 80%) must be ensured. However, when TiO (DPM).sub.2 is used as a Ti material, the coverage is at most about 50% for aspect ratio D/W of at least 3, whereby sufficient coverage is not obtained. As a result, it is difficult to form an upper electrode (a cell plate) on the BST thin film.